Method of gas carburizing

ABSTRACT

A method of gas carburizing is carried out at a carburizing temperature which is not higher than a peritectic point temperature at which δ iron and liquid phase are transformed into γ iron and not less than a eutectic point temperature at which liquid phase is transformed into γ iron and cementite. The method comprises the step of predetermining the limiting carburizing conditions at which the surface layer of a sample of a steel treatment object present in a carburizing atmosphere is austenitized without melting and the step of gas carburizing the treatment object under carburizing conditions which are set so as not to contradict the limiting carburizing conditions, at a carburizing temperature which is not higher than the peritectic point temperature and not less than the eutectic point temperature. The limiting carburizing conditions comprise an upper limit value of a partial pressure of carburizing gas at which the surface layer of the sample is austenitized without melting.

TECHNICAL FIELD

The present invention relates to a method for gas carburizing steelparts used, for example, in the automobile industry or machine industry.

BACKGROUND ART

When gas carburizing of steel treatment objects is carried out, thecarburizing treatment time can be shortened by raising the carburizingtemperature. However, if the carburizing temperature becomes too high,the treatment object melts down. For this reason, the carburizingtemperature that has been conventionally used in practice was less thanthe eutectic point temperature (in the case of an iron-carbonequilibrium diagram shown in FIG. 1, it is the C point temperature whichis 1147° C.) at which the liquid phase is transformed into γ iron andcementite, and usually was not higher than 1050° C.

In order to shorten the carburizing treatment time at the conventionalcarburizing temperature, the carburizing treatment was carried out tillthe concentration of carbon in the surface layer of the treatment objectbecame higher than the final target value, and then diffusion treatmentwhich caused the carbon in the surface layer of the treatment object todiffuse was carried out in a high-temperature atmosphere with a carbonconcentration less than that of the carburizing atmosphere, therebydecreasing the carbon concentration in the surface layer to the finaltarget value (Japanese Examined Patent Publication No. 1994-45868).

However, when the carburizing temperature is limited to a temperatureless than the eutectic point temperature, the diffusion rate of carbonatoms in the treatment object is restricted. Therefore, the carburizingtime cannot be shortened significantly. Thus, the problem is that thediffusion treatment requires a long time, thereby reducing productivity.

It is an object of the present invention to provide a method of gascarburizing capable of resolving the aforesaid conventional problems.

DISCLOSURE OF INVENTION

A method of gas carburizing according to an aspect of the presentinvention comprises the steps of predetermining limiting carburizingconditions at which the surface layer of a sample of a steel treatmentobject present in a carburizing atmosphere is austenitized withoutmelting at a carburizing temperature which is not higher than aperitectic point temperature at which δ iron and liquid phase aretransformed into γ iron and not less than a eutectic point temperatureat which liquid phase is transformed into γ iron and cementite; and gascarburizing the treatment object under carburizing conditions which areset so as not to contradict the limiting carburizing conditions, at acarburizing temperature which is not higher than the peritectic pointtemperature and not less than the eutectic point temperature, whereinthe limiting carburizing conditions comprise an upper limit value of apartial pressure of carburizing gas in the carburizing atmosphere atwhich the surface layer of the sample is austenitized without melting.Thus, the carburizing time is shortened by gas carburizing the treatmentobject which is austenitized by being heated to a highest possibletemperature. By increasing the carburizing gas concentration in thecarburizing atmosphere, the hardened layer of the treatment object canbe prevented from becoming shallow and have sufficient thickness withina short time.

The treatment object is austenitized when heated to a temperature abovethe GS line or ES line in the iron-carbon equilibrium diagram shown inFIG. 1. If the concentration of carbon in the surface layer of thetreatment object exceeds the JE line, melting is started in the surfacelayer. The present inventor has found out that an upper limit value of apartial pressure of carburizing gas at which the surface layer of thetreatment object is not melted exists under given carburizing conditionssuch as carburizing temperature and carburizing time, and that thisupper limit value decreases with the increase in carburizing temperatureand increases with the decrease in carburizing time. Because thecarburizing temperature range in the method of gas carburizing inaccordance with the present invention is from not higher than theperitectic point temperature (1494° C.) to not less than the eutecticpoint temperature (1147° C.), the carburizing time can be greatlyreduced. Moreover, setting the partial pressure of carburizing gas tonot higher than the predetermined upper limit value makes it possible tocarry out carburizing at a high temperature without causing melting inthe surface layer of the treatment object. As a result, the amount ofconsumed energy can be greatly reduced and energy can be saved.Furthermore, the gas carburizing process can be carried out in serieswith other processes, for example, machining processes or different heattreatment processes.

It is preferred that the limiting carburizing conditions comprise anupper limit value of carburizing temperature and an upper limit value ofcarburizing time at which the surface layer of the sample isaustenitized without melting, the relationship between the upper limitvalue of the partial pressure of carburizing gas, the upper limit valueof carburizing temperature, and the upper limit value of carburizingtime is predetermined, and the partial pressure of carburizing gas,carburizing temperature, and carburizing time are set such that thecarburizing conditions of the treatment object do not contradict thelimiting carburizing conditions which satisfy the predeterminedrelationship. The upper limit value of the partial pressure ofcarburizing gas, the upper limit value of carburizing temperature, andthe upper limit value of carburizing time serving as the limitingcarburizing conditions are interrelated and one of the upper limitvalues can be found by fixing the other two conditions among the partialpressure of carburizing gas, carburizing temperature, and carburizingtime. As a result, the conditions providing for the fastest possiblecarburization can be easily set within a range in which the surfacelayer of the treatment object is not melted.

A method of gas carburizing according to another aspect of the presentinvention is characterized in that when gas carburization of a steeltreatment object is carried out, a carburizing temperature is set at atemperature which is not higher than a peritectic point temperature (itis the J point temperature which is 1494° C. in the case shown inFIG. 1) at which δ iron and liquid phase are transformed into γ iron andnot less than a eutectic point temperature (it is the C pointtemperature which is 1147° C. in the case shown in FIG. 1) at whichliquid phase is transformed into γ iron and cementite, a target value ofcarbon concentration in the surface of the treatment object is set at avalue which is not higher than a value at which the surface of thetreatment object is not melted at the set carburizing temperature, and apartial pressure of carburizing gas in the carburizing atmosphere is setat a value at which the carbon concentration in the surface of thetreatment object can reach the set target value as a result of gascarburization carried out during a preset time. The present invention isbased on the discovery that when the carburizing temperature is set at atemperature which is not higher than the peritectic point temperatureand is not less than the eutectic point temperature and the target valueof carbon concentration in the surface of the steel treatment object isset at a value which is not higher than the value at which the surfaceof the treatment object is not melted at the set carburizingtemperature, if the partial pressure of carburizing gas in thecarburizing atmosphere is set at an appropriate value, then the carbonconcentration in the surface of the treatment object can reach the settarget value as a result of gas carburization and a sufficientcarburizing depth can be obtained. Because carburization at a hightemperature increases the movement rate of carbon atoms in the course ofcarburization over that attained by the conventional technology, thehardened layer depth that required several hours to be attained by theconventional technology can be attained within a short time. Moreover anexcess increase in the concentration of carbon in the surface isprevented.

As a result, the carburizing time can be shortened significantly byraising the carburizing temperature. Moreover, because the concentrationof carbon in the surface layer of the treatment object does not exceedthe set target value, the carbon diffusion treatment process isunnecessary. As a result, productivity can be increased. Furthermore,the gas carburizing treatment process can be carried out in series withother heat treatment processes. Therefore, it is preferred that thetreatment object is cooled without carrying out a diffusion treatmentafter the gas carburization has been carried out. It is also preferredthat the treatment object is reheated after the cooling. The reheatingis carried out, for example, by induction heating. It is also preferredthat quenching treatment of the reheated treatment object is carriedout. Cooling for the quenching treatment is carried out, for example, byoil cooling or gas cooling. In terms of shortening the carburizing time,it is preferred that the carburizing temperature is set at 1200° C. orhigher. In this case, prior to implementing the method of gascarburizing, it is preferred that limiting carburizing conditions arepredetermined, those conditions are set such that the surface layer of asample of a steel treatment object present in a carburizing atmosphereis austenitized without melting at a carburizing temperature which isnot higher than the peritectic point temperature at which δ iron andliquid phase are transformed into γ iron and is not less than theeutectic point temperature at which liquid phase is transformed into γiron and cementite. The limiting carburizing conditions comprise theupper limit value of the partial pressure of carburizing gas in thecarburizing atmosphere at which the surface layer of the sample isaustenitized without melting. The treatment object is austenitized whenheated to a temperature above the GS line or ES line in the iron-carbonequilibrium diagram shown in FIG. 1. If the concentration of carbon inthe surface layer of the treatment object exceeds the JE line, meltingis started in the surface layer. An upper limit value of the partialpressure of carburizing gas at which the surface layer of the treatmentobject is not melted exists under given carburizing conditions such ascarburizing temperature and carburizing time. Predetermining thelimiting carburizing conditions comprising the upper limit value of thepartial pressure of carburizing gas facilitates setting the partialpressure of carburizing gas and carburizing time.

In the present invention, the total pressure of carburizing atmospherecan be a normal pressure, or can be decreased or increased with respectto the normal pressure. The entire carburizing atmosphere can be acarburizing gas, or a gas mixture of a carburizing gas and a dilute gascan be used as the carburizing atmosphere. When a dilute gas is used,dilution is preferably carried out with an inert gas such as nitrogengas or argon gas. No specific limitation is placed on the type of steelof the treatment object which is subjected to gas carburizing by themethod according to the present invention, and the method of the presentinvention is applicable to any steel provided that it can beaustenitized at a temperature which is not higher than the peritecticpoint temperature and not less than the eutectic point temperature. Thissteel can be not only a carbon steel but also an alloy steel.

In the present invention, it is preferred that heating of the treatmentobject and a sample thereof is carried out with means capable ofhigh-speed heating of the surface layer thereof. The heating ispreferably carried out, for example, by induction heating or laserheating. As a result, heating efficiency of the carburization object canbe increased. Furthermore, because the carburizing treatment issimplified, quality control is facilitated. Thus, because the number offactors affecting the quality is small, even if quality problems such asspots, strains, or cracks in the treatment object are encountered, thecauses thereof can be easily clarified. Furthermore, a wall covering thecarburizing treatment space can be a cold wall and a waste gascombustion apparatus is unnecessary; therefore, degradation of workingconditions is prevented and initial investment is reduced, moreover themethod is applicable to single-item and small-scale production and canbe easily incorporated into a production line, for example, in-linetreatment of the individual production can be carried out. Because theconventional carburizing treatment furnace equipped with thermallyinsulating walls is not required, furnace heating or seasoning becomeunnecessary and running cost can be reduced.

In the present invention, the gas carburizing is preferably carried out,while causing a carburizing atmosphere comprising the carburizing gas ata constant partial pressure to flow. As a result, a constant partialpressure of carburizing gas can be maintained and treated products ofuniform quality can be obtained.

The method of gas carburizing according to the present invention cangreatly improve productivity.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an iron-carbon equilibrium diagram.

FIG. 2 illustrates the state of heating a sample of the treatment objectby the gas carburizing apparatus in an embodiment of the presentinvention.

FIG. 3 illustrates an example of relationship between the upper limitvalue of partial pressure of carburizing gas, different carburizingtemperatures and carburizing times.

FIG. 4 illustrates the state of heating a treatment object with the gascarburizing apparatus in an embodiment of the present invention.

FIG. 5 illustrates the relationship between the carburizing treatmenttime till the surface of the treatment object starts melting at acarburizing temperature of 1573 K, partial pressure of carburizing gas,and concentration of carbon in the surface.

FIG. 6 illustrates the relationship between the carburizing treatmenttime till the surface of the treatment object starts melting at acarburizing temperature of 1623 K, partial pressure of carburizing gas,and concentration of carbon in the surface.

FIG. 7 shows the relationship between the hardness and the distance(depth) from the surface of the treatment object carburized with acarburizing time of 1 min at a carburizing temperature of 1250° C. in anexample of the present invention.

FIG. 8 shows the relationship between the hardness and the distance(depth) from the surface of the treatment object carburized with acarburizing time of 1 min at a carburizing temperature of 1300° C. in anexample of the present invention.

FIG. 9 shows the relationship between the hardness and the distance(depth) from the surface of the treatment object carburized with acarburizing time of 10 min at a carburizing temperature of 1250° C. inan example of the present invention.

FIG. 10 shows the relationship between the hardness and the distance(depth) from the surface of the treatment object carburized with acarburizing time of 10 min at a carburizing temperature of 1300° C. inan example of the present invention.

FIG. 11 is a microphotograph as a substitution for a drawingillustrating the metallurgical structure prior to quenching in thesurface layer of the gas-carburized treatment object.

FIG. 12 is a microphotograph as a substitution for a drawingillustrating the metallurgical structure after quenching in the surfacelayer of the gas-carburized treatment object.

FIG. 13 illustrates the relationship between the concentration of carbonand the distance (depth) from the surface of the treatment objectobtained in an example of the present invention.

FIG. 14 illustrates an example of the relationship between thecarburizing time, diffusion time, and concentration of carbon in thetreatment object that was treated by the conventional carburizingmethod.

BEST MODE FOR CARRYING OUT THE INVENTION

The gas carburizing apparatus which is an embodiment of the presentinvention shown in FIG. 2 comprises a vacuum container 1, a heatingdevice 2, a vacuum pump 3 for reducing the pressure inside the vacuumcontainer 1, and a gas source 4 for supplying gas for carburizingatmosphere into the vacuum container 1. In the present embodiment, theheating device 2 carries out induction heating inside the vacuumcontainer 1 with a coil 2 a connected to a power source 7. The output ofthe power source 7 to the coil 2 a is variable.

First, in order to carry out gas carburization of a sample 5′ of a steeltreatment object, a thermocouple 6 is welded as a sensor for temperaturedetection to the surface layer of the sample 5′ set in the heatingdevice 2. Then, the pressure inside the vacuum container 1 is reduced byevacuating the vacuum container 1 with the vacuum pump 3. At this time,the pressure inside the vacuum container 1 is preferably about 27 Pa orless. Means for detecting the temperature is not limited to athermocouple.

After such pressure reduction, a gas for carburizing atmosphere isintroduced from the gas source 4 into the vacuum container 1. As aresult, the vacuum container 1 is filled with the carburizing atmosphereand the total pressure of the carburizing atmosphere is raised. Forexample, the pressure of the carburizing atmosphere inside the vacuumcontainer 1 is raised to about 80 kPa. The carburizing atmosphere iscomposed of a carburizing gas and a dilute gas. No specific limitationis placed on the type of the carburizing gas or dilute gas. Thecarburizing gas of the present embodiment is methane gas and the dilutegas is nitrogen gas. Using a hydrocarbon gas as the carburizing gasmakes it possible to realize a non-oxidizing carburization. Thecarburizing gas is not limited to hydrocarbon gases. The carburizingatmosphere may also be composed only of a carburizing gas.

In order to maintain a constant total pressure of carburizing atmosphereinside the vacuum container 1, the gas for carburizing atmosphere issupplied from the gas source 4 into the vacuum container 1 at a constantflow rate, and the gas for carburizing atmosphere is released by thevacuum pump 3 at a constant flow rate. As a result, the gas forcarburizing atmosphere flows inside the vacuum container 1 at a constantflow rate of, for example, 0.5 L/min, and the total pressure of thecarburizing atmosphere is maintained at, for example, about 80 kPa.Thus, the carburizing atmosphere containing a carburizing gas at aconstant partial pressure flows inside the vacuum container 1. Thepartial pressure of carburizing gas is a value obtained by multiplyingthe total pressure of carburizing atmosphere inside the vacuum container1 by a molar fraction or volume percent of the carburizing gas.Therefore, the set value of the partial pressure of carburizing gas canbe adjusted by changing the total pressure of carburizing atmosphereinside the vacuum container 1 or by changing the flow rate ratio of thecarburizing gas and dilute gas.

Then, the sample 5′ is heated with the heating device 2 to a setcarburizing temperature. The carburizing temperature is not higher thanthe peritectic point temperature at which δ iron and liquid phase aretransformed into γ iron and not less than the eutectic point temperatureat which liquid phase is transformed into γ iron and cementite. The setvalue of the carburizing temperature can be adjusted by changing theoutput of the heating device 2 to the coil 2 a.

When the sample 5′ is gas carburized by holding for the set carburizingtime under the set partial pressure of carburizing gas and the setcarburizing temperature, it is checked to see whether the surface layerof the sample 5′ is melted or not.

If the surface layer of the sample 5′ has not melted during thecarburization, another carburization of another sample 5′ is carried outby increasing the set value of the partial pressure of carburizing gas.If the surface layer of the sample 5′ has melted, another carburizationof another sample 5′ is carried out by decreasing the set value of thepartial pressure of carburizing gas. By repeating this process, theupper limit value of the partial pressure of carburizing gas ispredetermined as a limit carburizing condition at which austenitizationis conducted without melting the surface layer of the sample 5′.

A method for predetermining the upper limit value of the partialpressure of carburizing gas by fixing the carburizing temperature andcarburizing time was described hereinabove, but this method can beappropriately modified without departing from the essence of the presentinvention. Thus, the upper limit value of carburizing time can be foundby fixing the partial pressure of carburizing gas and carburizingtemperature, or the upper limit value of carburizing temperature can befound by fixing the partial pressure of carburizing gas and carburizingtime.

FIG. 3 shows an example of the relationship between the upper limitvalue of the partial pressure of carburizing gas, the upper limit valueof carburizing temperature, and the upper limit value of carburizingtime predetermined in the above-described manner. In the presentembodiment, the concentration of methane (vol %) in carburizingatmosphere corresponding to the partial pressure of carburizing gas isplotted on the ordinate in FIG. 3, but the partial pressure ofcarburizing gas may also be represented by another. For example, when nodilute gas is used, the partial pressure of carburizing gas is equal tothe total pressure of carburizing atmosphere, and therefore the methaneflow rate can be used to represent the partial pressure of carburizinggas. By using a similar method it is possible to find not only the linesfor 1 min, 3 min, and 30 min shown in FIG. 3, but also any line or curvelocated therebetween. As shown in FIG. 3, the upper limit value of thepartial pressure of carburizing gas in the carburizing atmospheredecreases with the increase in carburizing temperature and increaseswith the decrease in carburizing time. Further, the relationship shownin FIG. 3 is merely an example, and this relationship differs dependingon the arrangement of structural components of heating device 2,material and arrangement of sample 5′ to the heating device 2, type ofheating device 2, type and flow rate of carburizing gas, and the like.

Gas carburization of a steel treatment object 5 is thereafter carriedout by using the above-described apparatus for gas carburization underthe carburizing conditions which are set so as not to contradict thelimiting carburizing conditions satisfying the predeterminedrelationship. Carburization of the steel treatment object 5 can becarried out in the same manner as carburization of the sample 5′.

To be more precise, as shown in FIG. 4, the steel treatment object 5 isset to the heating device 2, the air present inside the vacuum container1 is evacuated with the vacuum pump 3, a gas for carburizing atmosphereis introduced into the vacuum container 1 from the gas source 4 to raisethe pressure of the carburizing atmosphere to the set pressure, the gasfor carburizing atmosphere is supplied at a constant flow rate from thegas source 4 into the vacuum container 1, and the gas for carburizingatmosphere is released at a constant flow rate with the vacuum pump 3.As a result, the partial pressure of carburizing gas in the carburizingatmosphere inside the vacuum container 1 is set at not higher than theupper limit value that was predetermined as a limiting carburizationcondition. Then, the steel treatment object 5 is heated to thecarburizing temperature with the heating device 2. The carburizingtemperature is set at a value which is not higher than the aforesaidperitectic point temperature and not less than the eutectic pointtemperature. The carburizing temperature during heating of the sample 5′can be reproduced during heating of the treatment object 5 bycontrolling the heating device 2 in the same manner as during heating ofthe sample 5′, and therefore, it is not necessary to weld thethermocouple 6 to the treatment object 5. Gas carburization is carriedout by holding the treatment object 5 for the set carburizing time underthe set partial pressure of carburizing gas and set carburizingtemperature.

For example, when gas carburization of the steel treatment object 5 iscarried out, the carburizing temperature is set at a temperature whichis not higher than the peritectic point temperature at which δ iron andliquid phase are transformed into γ iron and not less than the eutecticpoint temperature at which liquid phase is transformed into γ iron andcementite. Furthermore, the target value of carbon concentration in thesurface of the treatment object 5 is set at a value which is not higherthan a value at which the surface of the treatment object 5 is notmelted at the set carburizing temperature. Further, the partial pressureof carburizing gas in the carburizing atmosphere is set at a value atwhich the carbon concentration in the surface of the treatment objectcan reach the set target value as a result of gas carburization carriedout during a preset period. The set value of the partial pressure ofcarburizing gas corresponding to the carburizing time can bepredetermined by experiments. The set values of the carburizing time andpartial pressure of carburizing gas are less than the aforesaid upperlimit values corresponding to the set temperature. Therefore, thesetting of the carburizing time and partial pressure of carburizing gasis facilitated by predetermining the aforesaid upper limit values.

For example, because the surface of the treatment object 5 startsmelting at a carbon concentration of about 1.15 wt. % at a carburizingtemperature of 1573 K, the relationship between the carburizingtreatment time required for the surface of the treatment object 5 tostart melting at a carburizing temperature of 1573 K, the partialpressure of carburizing gas, and the concentration of carbon in thesurface is expressed by FIG. 5 from the relationship shown in FIG. 3.FIG. 5 demonstrates that when the carburizing temperature is set at 1573K and the target value of carbon concentration in the surface oftreatment object 5 is set at a concentration of less than 1.15 wt. % atwhich no melting occurs, the concentration of methane in the carbonatmosphere should be set at less than 10 vol % if the carburizing timeis 1 min, and the concentration of methane in the carbon atmosphereshould be set at less than 3 vol % if the carburizing time is 10 min.Furthermore, for example, since the surface of the treatment object 5starts melting at a carbon concentration of about 0.9 wt. % at acarburizing temperature of 1623 K, the relationship between thecarburizing treatment time required for the surface of the treatmentobject 5 to start melting at the carburizing temperature of 1623 K, thepartial pressure of carburizing gas, and the concentration of carbon inthe surface is shown by FIG. 6 from the relationship shown in FIG. 3.FIG. 6 demonstrates that when the carburizing temperature is set at 1623K and the target value of carbon concentration in the surface of steeltreatment object 5 is set at a concentration of less than 0.9 wt. % atwhich no melting occurs, the concentration of methane in the carbonatmosphere should be set at less than 1.8 vol % if the carburizing timeis 1 min, and the concentration of methane in the carbon atmosphereshould be set at less than 0.8 vol % if the carburizing time is 10 min.Gas carburizing is carried out by holding the treatment object 5 for aset carburizing time under the aforesaid set partial pressure ofcarburizing gas and set carburizing temperature. Once the setcarburizing time has elapsed, the carburizing is stopped by stopping thesupply of carburizing gas or terminating the heating with the heatingdevice 2.

With the method of gas carburizing according to the present invention,the carburizing time can be greatly shortened because the carburizingtemperature range is set between not higher than the peritectic pointtemperature and not less than the eutectic temperature. Moreover,setting the partial pressure of carburizing gas to not higher than thepredetermined upper limit value makes it possible to carry out thecarburization at a high temperature without melting the surface layer ofthe steel treatment object 5. For example, the diffusion coefficient ofcarbon atoms in y iron is 3.59×10,5 mm²/sec at a temperature of 1000°C., but increases to ten or more times, that is, 43×10⁻⁵ mm²/sec at atemperature of 1300° C. Thus, the migration speed of carbon atoms at atemperature of 1300° C. is not less than tenfold that at a temperatureof 1000° C. Therefore, the time required to obtain the desiredcarburization depth can be greatly reduced and the usual carburizationdepth can be obtained at a carburizing time of about 1 to 10 min.Moreover, because the concentration of carbon in the surface layer ofthe treatment object 5 does not exceed the set target value, a carbondiffusion treatment step becomes unnecessary. As a result, thecarburizing treatment time can be greatly shortened and the productivitycan be increased. Furthermore, the gas carburization treatment step canbe carried out in series with other heat treatment steps. Setting thecarburizing temperature at not less than 1200° C. is preferred from thestandpoint of shortening the carburizing time, and this temperature canbe set at not less than 1300°. Furthermore, because gas carburization iscarried out while causing the carburizing atmosphere comprising thecarburizing gas at a constant partial pressure to flow inside the vacuumcontainer 1, a constant partial pressure of carburizing gas can bemaintained and the uniformity of the quality of treatment object 5 canbe improved. Moreover, no soot generation was observed in thecarburizing treatment implemented according to the present invention,and in this respect, too, the present invention is greatly superior tothe conventional vacuum carburizing.

Once the aforesaid gas carburizing has been completed, the treatmentobject 5 is cooled without carrying out the diffusion treatment. Nospecific limitation is placed on the cooling method, and natural coolingor a variety of forced cooling methods can be used. Furthermore, thetreatment object 5 subjected to gas carburization is preferably quenchedby reheating after cooling and then rapidly cooling. A secondaryquenching may also be carried out by employing the primary cooling as arapid cooling. The atmosphere for carrying out the quenching ispreferably a neutral protective atmosphere, that is, the atmosphere inwhich the treatment object is neither carburized nor decarburized atthis temperature, but the treatment can be carried out in anotheratmosphere of such as inert gas. The reheating temperature for quenchingis set at not less than the temperature at which at least the surfacelayer of the treatment object 5 is austenitized above the GS line or ESline in the equilibrium diagram shown in FIG. 1.

EXAMPLE 1

With the method of gas carburizing of the above-described embodimentaccording to the present invention, the limiting carburizing conditionswere predetermined and gas carburization was carried out under thecarburizing conditions that are set so as not to contradict the limitingcarburizing conditions. The treatment object 5 had a shape of rightcylinder with a diameter of 10 mm and a length of 52 mm made of anickel-chromium-molybdenum steel (Japanese Industrial Standard SNCM420).In the present example, the carburized treatment object 5 was naturallycooled inside the vacuum container 1, hardened, polished, and finishprocessed with a diamond paste with a particle size of 3 micrometers,followed by hardness measurements and structure observations. The vacuumcontainer 1 was purged prior to the carburization. The carburizing gaswas methane and the dilute gas was nitrogen. During carburization, thegas for carburizing atmosphere was caused to flow inside the vacuumcontainer 1 at a constant flow rate of 0.5 L/min. Hardening was carriedout by holding the treatment object 5 for 10 min inside a quartz tubefurnace kept at a temperature of 860° C. in which the nitrogen gasatmosphere was flowed and then quenched into oil. It goes without sayingthat furnaces of other types, including the induction heating furnaces,can be used for the hardening.

FIG. 7 shows the relationship between the hardness (Hv) and the distance(depth) (mm) from the surface of the treatment object 5 that was gascarburized under the following carburizing conditions: carburizingtemperature 1250° C., carburizing time 1 min, and concentrationcorresponding to partial pressure of methane, which is the carburizinggas, 40 vol %. In this case, the target value of carbon concentration inthe surface of the treatment object 5 was set at 1.4 wt. %.

FIG. 8 shows the relationship between the hardness (Hv) and the distance(depth) (mm) from the surface of the treatment object that was gascarburized under the following carburizing conditions: carburizingtemperature 1300° C., carburizing time 1 min, and concentration ofmethane, which is the carburizing gas, 10 vol %. In this case, thetarget value of carbon concentration in the surface of the treatmentobject 5 was set at 1.15 wt. %.

FIG. 9 shows the relationship between the hardness (Hv) and the distance(depth) (mm) from the surface of the treatment object 5 that was gascarburized under the following carburizing conditions: carburizingtemperature 1250° C., carburizing time 10 min, and concentrationcorresponding to partial pressure of methane, which is the carburizinggas, 10 vol %. In this case, the target value of carbon concentration inthe surface of the treatment object 5 was set at 1.4 wt. %.

FIG. 10 shows the relationship between the hardness (Hv) and thedistance (depth) (mm) from the surface of the treatment object that wasgas carburized under the following carburizing conditions: carburizingtemperature 1300° C., carburizing time 10 min, and concentration ofmethane, which is the carburizing gas, 3 vol %. In this case, the targetvalue of carbon concentration in the surface of the treatment object 5was set at 1.15 wt. %.

FIGS. 7 through 10 confirm that the carburized layer with a sufficienteffective carburization depth can be obtained within a carburizing timeof 1 through 10 min. Further, the partial pressure of methane, which isa carburizing gas, is obtained by multiplying the total pressure ofcarburizing atmosphere by the methane concentration. In the exampleshown in FIGS. 7 through 10, the total pressure of carburizingatmosphere was about 80 kPa.

The metal structure prior to quenching of the surface layer of thetreatment object 5 subjected to gas carburizing at a carburizingtemperature of 1300° C. for a carburizing time of 1 min in accordancewith the present invention is shown in FIG. 11. The metallurgicalstructure after quenching is shown in FIG. 12. A coarse structure thatwas present prior to quenching is not present after the quenching, whichconfirms that quenching makes the coarse structure fine.

EXAMPLE 2

Gas carburization was carried out under the carburizing conditions thatwere set by the gas carburization method of the above-describedembodiment of the present invention. The treatment object 5 had a shapeof right cylinder with a diameter of 10 mm and a length of 52 mm made ofa nickel-chromium-molybdenum steel (Japanese Industrial StandardSNCM420) as used in the above-described embodiment. The carburizedtreatment object 5 was naturally cooled inside the vacuum container 1,without being subjected to diffusion treatment, then was hardened,polished, and finish processed with a diamond paste with a particle sizeof 3 micrometers. The vacuum container 1 was purged prior to thecarburization. The carburizing gas was methane and the dilute gas wasnitrogen. Hardening was carried out by holding the treatment object 5for 10 min inside a quartz tube furnace kept at a temperature of 860° C.in which nitrogen gas atmosphere was flowed and then quenched into oil.It goes without saying that furnaces of other types, including theinduction heating furnaces, can be used for the hardening. Thecarburizing temperature was 1300° C., the carburizing time was 1 min,the concentration corresponding to the partial pressure of methane,which is the carburizing gas, in the carburizing atmosphere was 10 vol%, the target value of carbon concentration in the surface of thetreatment object 5 was 0.74 wt. %, the total pressure of carburizingatmosphere was 80 kPa, and the gas for carburizing atmosphere was flowedinside the vacuum container 1 during the carburization at a constantflow rate of 0.5 L/min. FIG. 13 shows the relationship between theconcentration of carbon (wt. %) and the distance (depth) (mm) from thesurface of the treatment object 5 obtained in the present example. Inthe present example, the depth to which the concentration of carbon washigher than 0.2 wt. % of the base metal, that is, the completecarburizing depth, was 0.9 mm, thereby confirming that the sufficienteffective carburizing depth can be obtained without employing adiffusion treatment step. Furthermore, the results obtained were matchedwell with the results on the complete carburizing depth obtained fromFIG. 8.

According to the above-described examples, the concentration of carbonin the surface of the treatment object 5 can be brought to the targetvalue and a sufficient carburizing depth can be obtained withoutemploying a diffusion treatment. By contrast, FIG. 14 shows an exampleof the relationship between the concentration of carbon in the surfaceand carburizing time in the treatment object obtained by theconventional carburizing method, in which because the concentration ofcarbon exceeds the target value when only the carburizing treatment iscarried out, a subsequent diffusion treatment has to be carried out.

The present invention makes it possible to shorten significantly thecarburizing time by raising the carburizing temperature. Moreover,because the concentration of carbon in the surface layer of thetreatment object 5 does not exceed the set target value, the carbondiffusion treatment becomes unnecessary and productivity can beincreased.

The present invention is not limited to the above-described embodimentsand examples and can be modified variously within the scope of thepresent invention.

1. A method of gas carburizing comprising the steps of: predetermininglimiting carburizing conditions at which the surface layer of a sampleof a steel treatment object present in a carburizing atmosphere isaustenitized without melting at a carburizing temperature which is nothigher than a peritectic point temperature at which δ iron and liquidphase are transformed into γ iron and not less than a eutectic pointtemperature at which liquid phase is transformed into γ iron andcementite; and gas carburizing the treatment object under carburizingconditions which are set so as not to contradict the limitingcarburizing conditions, at a carburizing temperature which is not higherthan the peritectic point temperature and not less than the eutecticpoint temperature, wherein the limiting carburizing conditions comprisean upper limit value of a partial pressure of carburizing gas in thecarburizing atmosphere at which the surface layer of the sample isaustenitized without melting.
 2. The method of gas carburizing accordingto claim 1, wherein the limiting carburizing conditions comprise anupper limit value of carburizing temperature and an upper limit value ofcarburizing time at which the surface layer of the sample isaustenitized without melting, the relationship between the upper limitvalue of the partial pressure of carburizing gas, the upper limit valueof carburizing temperature, and the upper limit value of carburizingtime is predetermined, and the partial pressure of carburizing gas,carburizing temperature, and carburizing time are set as the carburizingconditions of the treatment object so as not to contradict the limitingcarburizing conditions which satisfy the predetermined relationship. 3.A method of gas carburizing comprising gas carburizing a steel treatmentobject, wherein a carburizing temperature is set at a temperature whichis not higher than a peritectic point temperature at which δ iron andliquid phase are transformed into γ iron and not less than a eutecticpoint temperature at which liquid phase is transformed into γ iron andcementite; a target value of carbon concentration in the surface of thetreatment object is set at a value which is not higher than a value atwhich the surface of the treatment object is not melted at the setcarburizing temperature; and a partial pressure of carburizing gas in acarburizing atmosphere is set at a value at which the carbonconcentration in the surface of the treatment object can reach the settarget value as a result of gas carburization carried out during apreset time.
 4. The method of gas carburizing according to claim 3,wherein the treatment object is cooled without carrying out a diffusiontreatment after the gas carburization has been carried out.
 5. Themethod of gas carburizing according to claim 4, wherein the treatmentobject is reheated after the cooling.
 6. The method of gas carburizingaccording to claim 5, wherein hardening of the reheated treatment objectis carried out.
 7. The method of gas carburizing according to any ofclaims 3 to 6, wherein the is set at 1200° C. or higher.
 8. The methodof gas carburizing according to any of claims 1 to 6, wherein said gascarburizing is carried out, while causing the carburizing atmospherecomprising the carburizing gas at a constant partial pressure to flow.